Process for the recovery of fixed nitrogen when using or treating combustibles



Patented Feb. 10, 1925 WALTER OS'IWALD, 013TANNDORF-ONj-THE-MULDE. GERMANY.

PROCESS FOR THE RECOVERY OF FIXED NITROGEN WHEN COMTBUSTIELES;

To all whom it may concern Beit' known that I, WALTER UsTWALn, a citizen of Germany, residing at Tanndorfon tlieiM ulde, Saxony, Germ-any, have invented certain new and useful Improvemer ts in Processes forv the Recovery of Fixed Nitrogen when Using or Treating Combustihles (for which I (have filed applications in Germany, March 2st, 1917, in Italy,fJuly 1920 and in spain, July 7, 1920); and Ido hereby declare the following to be a full, clear, and. exactdescription of the invention, of which the following is a specification.

All combustibles contain more or less amounts offixed nitrogen. When distilling combustibles and especially pit coal incoke ovens or gas retorts hitherto about 15% of thenitrogen contained in the coalhas been recovered with the gas liquor. About50% remains as s'o called fixed nitrogen in the coke and tar. The remaining part of the fixed nitrogen decomposes at the prevailing hightemperature to worthless free nitrogen.

WVhen distilling combustibles in generators the fixed nitrogen as a ruleis seldom recovered at all and it chiefly burns to worthless free nitrogen. Only if according to Mond the double ortreble amount of steam in weight is forced through the generator thus lowering the temperature of the firing zone to a very great extent and which is done an the expense of the quality of the gas obtained and of the eiliciency offthe plant, it will be possible to recover as much as of fixed nitrogen from the combustible.

Heretofore it has not been possible to sue cessfully recover the fixed nitrogen from blast furnaces.

It hasalso hitherto not been possible to guard the fixed nitrogen of the combustible against the destructive effect of the high temperature of the flames during combustion.

The gas liquor containing the fixed nitrogeniand obtained with the distilling and gasifying of the combustible represents a crude mixture of various nitrogen compounds and especially of ammonia compounds. A distinction is made between uniixed ammonia which can be driven off by simply heating the liquor and to which in addition to the free iaimnonia "(Ni -I 'am- I'I onium sulphide (NIL) QS'etc. ammonium Application filed August 23, 1921. Serial No. 494,695.

carbonate (NIL) 2G0 etc. must be counted andfixed ammonia from which the an monia can only be liberated by drivin'g'it' oil with steam and lime andwith'which ammonium sulphate, ammonium"chloride,' ammonium sulphocyanate, ammonium thiosulphate, ammonium sulphitehave tofbe classed.

Since naturally this varied mixture can, in exceptional cases only be furtl'rer dealt with pa rtlyalsofor its sing in the liquid state, the gasliquor is as a rule treated with the object ofobtaining a uniforln'amnmnium salt viz, generally of producing ainmoniiun sulphate. This is effected by treating the gas liquor with lime and steam in special vessels, whereby ammonia, carbonic acid,sulphurettedhydrogen and other gases are liberated. These gases are then passed into sulphuric acid whereby the latter will fix the ammonia however allowing most of the other gases to flbw through undisturbed. The Well known disadvantages of this process are the following:

(a) the great and useless consumption of sulphuric acid,

(blthe very obnoxious Waste water of the driving-oft chambers,

(0) the consumption of lime, (d) the very obnoxious Waste gases, (e) the apparatusnnd the working of same being expensive and complicated. .Acco-rding to the present invention the total by-product recovery of the fixed nitrogen from the coinbustibles ispIacedupOn another basis by intentionallyworking'for a uniform kind of nitrogen compounds instead of for acrude mixture of all possible nitrogen compounds. This one uniform nitrogen compound has also the advantage of giving a considerably greater yield of nitrogenby the decomposition of this nitrogen as a protecting substance preveiiting'the burning of the ammonia and theyieldiiigof free nitrogen. Thus the present invert: tion does notonly enable theuse of considerably simplifiedapparatus and of a considerably simplified process forthe recovery of byproducts but in addition thereto'gives a considerably greater yield! L o The present invention consists therein that fixed nitrogen of the combustible is changed on the spot and in the moment of its being liberated from s the I combustible into 1 an ammonium salt", which (12) is volatile-1S0 that USING on rnnA r no.

full

the fixed nitrogen (3) passes away with the gas in a uniform state and can be recovered.

This is for instance effected by blowing gaseous hydrochloric acid into the grate of a generator. The hydrochloric acid absorbs all arising ammonia compounds and in consequence of its being a strong acid changes them into ammonium chloride which owing to its complete volatility at 450 degrees will go off together with the gases. At the same time however the hydrochloric acid will react with all other nitrogen compounds which may arise and will change them into ammonia or ammonium chloride respectively. Thus it increases the yield of nitrogen compounds.

While, as it is well known, ammonia when passing through red hot chambers is quickly decomposed to free nitrogen, ammonium chloride will flow through higa and apparently the highest temperatures without becoming decomposed. This surprising result may be explained thereby that the strong hydrochloric acid prevents at high temperatures tae splitting effect of the heat to take place on the ammonia molecule. First the molecule of the ammonium chloride must be dissociate-d before the molecule of ammonia itself is affected.

All volatile acids, such as carbonic acid, sulphurous acid which may arise are weaker chemically than the hydrochloric acid and are therefore prevented by the latter to combine with ammonia. Nonvolatile acids cannot move away with the gas because they are not capable of becoming volatilized. This special effect of the hydrochloric acid exercises at the same time a peculiar protecting effect upon the ammonia. when the gas is led away through the regions of high temperatures, as soon as the gas together with the nitrogen compounds is driven out of the coal. At a temperature of about 250 ammonium sulphite and ammonium carbonate will be both decomposed completely into ammonia on the one hand and into carbonic acid or sulphurous acid respectively on the other hand. At this temperature however the dissociation of the ammonium chloride into hydrochloric acid and ammonia is not complete. In consequence of a surplus of hydrochloric acid in these zones the ammonia resulting from the ammonium sulphite and ammonium carbonate is instantaneously transformed into ammonium chloride by the hydrochloric acid and is in the first instance protected at a temperature up to approximately 500. But also at higher temperatures there exists notwithstanding agreater aliinity between the ammonia and hydrochloric acid than perhaps between the ammonia or other acids. The tendency of the ammonium chloride to form again is therefore much greater than for instance that of the ammonium sulphite.

Upon the grade of this aifinity however or upon the degree to which the ammonia is fixed to the anion in case of a complete dissociation depends only its possibility of being decomposed to form nitrogen and water. While in the absence of hydrochloric acid the high temperature after the carbonic acid and sulphurous acid have been driven off effect directly a dissociation or combustion of the ammonia, the presence of the very strong hydrochloric acid so to speak compels the temperature to employ the main bulk of the energy to overcome the strength of the hydrochloric acid. Not until the high temperature has overcome the affinity of hydrochloric acid-ammonia, can it break up the ammonia. The great aliinity thus acts so to speak as a means for reducing the influence of the temperature, just as Mond has attempted to effect this in quite a different way, viz, by cooling the firing zones and by blowing in great quantities of steam.

It was further found that when the hydrochloric acid was in the nascent state there was quite a particularly favourable effect which could be attributed to the increasing of its saponilying effect. Hydrochloric acid when in the nascent state may however be produced according to the present invention in a most simple and exceedingly cheap manner and timely and locally coinciding with the formation of the ammonia by mixing the combustibles with chlorides especially with those of the alkali metals and alkali earth metals of the earth. Strange to say they are completely dissociated by the steam which is present or which if required is,

added in a regulating manner so that at the same time ammonia compounds and hydrochloric acid arise each in the nascent state are found.

As to the combustion any kinds of coal containing nitrogen may be mixed with chlorides, for instance magnesium chloride obtained as a final solution from potash works or calcium chloride solution being the final solution in the ammonia-soda manufacture being poured over said coal. These solutions may however also be injected into the red hot zones of combustion. During combustion hydrochloric acid is liberated in the oven which supports the formation of ammonium chloride and thus prevents the ammoniafrom becoming dissociated so that a considerable yield of ammonia salt is obtained if the smoking gases are washed with water.

When making coke or in case of distilling the coal, for instance gaseous hydrochloric acid may be introduced into the retorts. The amount of hydrochloric acid added is so chosen that it approximately corresponds to half the amount of nitrogen contained in the coal, as the fixed nitrogen which amounts to approximately 50% of the total amount of nitrogen contained in the coal remains in the coke. The hydrochloric acid prevents the ammonia from being decomposed and is deposited with the gas liquor. Instead of introducing hydrochloric acid in a gaseous state, chlorides may be added in any desired combinations as in the first example (the combustion) of feeding coal.

In case of distillation the process may be carried out in such a manner that for instance hydrochloric acid is in the state of vapour passed into the zone of temperature at from 500 to 1000". In this case the quan tity of hydrochloric acid chosen is almost equivalent to the amount of nitrogen contained in the coal, as here a pyrogenous destruction of the coal will take place which also drives the non-fixed nitrogen from the coal. Also in this caseagain the hydro chloric acid can be added in the shape of chlorides instead of introducing it in the gaseous state.

In case of the blast furnace process for example red hot blast furnace coke is quenched by means of magnesium chloride for example in the form of a solution. The red hot coke absorbs this solution, evaporates the water and in this manner is impregnated with magnesium chloride. During the subsequent blast furnace process ammonia and hydrochloric acid are liberated through the action of the high tempera ture whereby the hydrochloric acid combines with the ammonia to form ammonium chloride, thereby preventing the ammonia from being burnt. It is equally possible to add chlorides and especially calcium chloride to the charge during the working of the blast furnace. According to the amount of lime, contained in the added calcium chloride, a saving of ordinary lime may be made with the charges.

It has furthermore been found that it suflices to maintain the presence of such quantities of hydrochloric acid which are equivalent to the amount of nitrogen which shall be recovered. By the addition of equivalent quantities approximately quantitative yields are obtained and thus the forma tion of acid washing solution is avoided.

A still larger yield is obtained if pro vision is made for a surplus of hydrochloric acid prevailing in the zone of formation of the ammonia which as long as the gas is still above its point of condensation will be neutralized again by the ammonia. It will be particularly advantageous if this ammonia is produced from working products in the upper part of the generator, by salting every tenth or twentieth charge with raw ammonium rhloride and lime instead of with chlorides or with working ashes containing lime. F or the same purpose an unsalted charge of coal may be added from time to time.

From the gas the ammonium chloride can easily be recovered as a sublimate or as a viscous mass containing tar by cooling it to a temperature of less than 450. The gases may also be washed with water whereby a uniform solution of ammonium chloride is obtained which when being concentrated gives sal ammoniac pure in a technical sense. The gas need hereby not be cooled completely in contradistinction to the extensive gas coolers and the reducing of the temperature to that of the room required for the ordinary recovery of gas liquor on account of the volatility of the free ammonia. A cool-- ing of the gas down to the boiling point of the solution will be perfectly sufficient. The solution may in particular he preferably made to circulate in the scrubber in such a manner that it is enriched up to the saturating point so that upon being cooled solid sal ammoniac will drop out.

If the ammonium chloride is recovered with tar it may easily be separated from the latter by grinding it with water and extracting the ammonium chloride with water. WVhile, when working it in a hot condition the result will be a smeary mass which can only be treated with difliculty, a clear solution of ammonium chloride and pulverized pitch will be obtained in this manner.

In this manner nearly the total amount of nitrogen contained in the combustible was recovered during gasification. In case of distillation the output also increased quite considerably and even the blast furnace and the combustion brought forth remarkably large outputs with the present process.

I claim as my invention 1. In a process for the recovery of nitrogen from gases derived from coal, subjecting the gases within the zone of their formation to hydochloric acid in the nascent state in quantity equivalent to the amount of nitorgen to be fixed, thereby fixing the nitrogen in the form of ammonium chloride and adding chlorides to an amount equivalent to the liberated amount of nitrogen of the coal.

2. In a process for the recovery of nitrogen from gases derived from coal, subjecting the gases within the zone of their formation to hydrochloric acid in the nascent state in quantity equivalent to the amount of nitrogen to be fixed, thereby fixing the nitrogen in the form of ammonium chloride and adding surplus amounts of chloride for the production of acid gases which react with the ammonia in the coal gases to form ammonium chloride.

In testimony whereof I have signed my name to this specification in the presence of a subscribing witness.

WALTER OSTWALD.

H. ADAM. 

